Field of the Invention
The present invention relates to a magnetic disk, a method of and an apparatus for manufacturing the same, and a magnetic disk unit obtained by the same. More particularly, the present invention pertains to a magnetic disk of the type which employs a thin magnetic film as a recording medium, which enables the floating characteristics of a magnetic head thereto to be improved, and which exhibits improved sliding-resistant characteristics thereof, a method of and an apparatus for manufacturing the same, and a magnetic disk unit obtained by the same.
In high-density and large-capacity magnetic disks, the magnetic recording medium formed on a non-magnetic disk substrate is conventionally a coated film formed by coating magnetic powders held together by a resin on the recording medium. However, use of this coated film has been giving way in recent years to use of a thin magnetic metal layer directly formed on the disk substrate by deposition or sputtering. When the magnetic disk units is driven, the magnetic disk (hereinafter referred to merely as "the disk") is housed in a magnetic disk units in a stationary state with a magnetic head (hereinafter referred to merely as "a head") having a specific load being elastically in contact and pressed against the surface of the disk. Then, the magnetic disk is normally driven by the contact start stop (abbreviated to CSS) method as follows: as the disk starts rotating, the head starts sliding against the surface of the disk. Once the rotational speed of the disk reaches a value as high as 1000 rpm i.e. high speed rotation, the head floats in the air at a predetermined distance from the surface of the disk due to the dynamic pressure effects caused by the air flow generated between the sliding surface of the head and the disk. The magnetic head unit is arranged such that the head can be freely moved in the radial direction of the disk in this floating state so as to allow data to be stored on and retrieved from the surface of the disk at a desired position. When the operation of the disk drive is to be stopped, the rotational speed of the disk decreases, and thus the head starts sliding against the surface of the disk again. The head then stops in a state in which it is in contact with and presses against the disk. In this CSS driving method, each time the head and disk assembly is driven, the sliding surface of the head repeats a cyclic operation, consisting of stopping, sliding against the surface of the disk, floating in the air, sliding against the surface of the disk, and stopping. In order to facilitate floating of the head, the surface of the disk is generally provided with micro grooves which extend in the circumferential direction thereof. FIG. 10 shows a section of a disk 80. When the grooves of this type are to be formed, the surface of a disk substrate 30 is subjected to the surface polishing process called texture process prior to the formation of the magnetic film. Thereafter, a thin magnetic layer 32, a protective film 33 and a lubricating film (not shown) are formed on the irregular surface of the substrate, by means of which the grooves formed in the surface of the disk substrate are reproduced on the surface of the disk.
The texture process to which the disk surface is subjected is a polishing technique essential to the magnetic disk which is driven by the CSS driving method. For example, Japanese Patent Unexamined Publication No. 62-219227 discloses that, when the disk surface is polished to a maximum surface roughness of 0.02 to 0.1 .mu.m, the thickness of a non-magnetic metal film (Cr film) can be reduced and productivity can thus be improved. It has also been disclosed that, when the CSS test was conducted 20,000 times on such a magnetic disk, no damage occurred on the surface of the disk while head crashing easily occurred on the surface of the disk having a surface roughness of 0.1 .mu.m or above, and that, when the disk surface was not subjected to the texture process, it was damaged and head crashing occurred when CSS was conducted 5000 times.
FIG. 12 shows a conventional texture processing device which is disclosed in Japanese Patent Unexamined Publication No. 54-23294. In this device, the two surfaces of the disk substrate 30 are simultaneously processed by pressing polishing tapes 4, which are moved in a vertical direction by the rotation of reels 6, in the direction indicated by the arrow toward the two surfaces of the rotating disk substrate 30 by means of contact rollers 8 while moving back and forth in the radial direction of the disk substrate. FIGS. 7 and 8 are front and side elevational views showing the positional relation between the substrate 30 and the polishing tapes 4 which are moving along the substrate 30. In this texture process, a micro groove 37 such as that shown in FIG. 11, can be formed by the polishing tape. However, formation of the groove causes an unstable rising portion 36 to be formed at the shoulder of the groove. The rising portion 36 remains on the surface of the disk as a fine protrusion.
Hence, it has been proposed to conduct on the surface of the disk substrate a first polishing process which is a normal polishing process and then a second polishing process which employes abrasive grains smaller than those employed in the first polishing process to remove only the protrusions generated on the surface of the substrate by the first polishing process without removing the micro grooves formed in the surface of the substrate. Such a technique is disclosed in Japanese Patent Unexamined Publication No. 62-248133.
The maximum surface roughness of the disk substrate on which micro grooves are formed by the texture process and protrusions formed on the substrate in order to achieve improvement in the head floating have been specified. However, the optimal conditions of the surface which is subjected to the texture process in terms of the CSS characteristics and head adhesion characteristics are unknown, and the problems involving head crashing or the like have not yet been solved.
As high-density and large-capacity magnetic disks have been developed in recent years, the distance which the head floats above the surface of the disk in the CSS drive is becoming shorter and shorter. For example, it is required under severe condition that the gap between the surface of the disk and the sliding surface of the head (which is the distance by which the head floats above the surface of the disk), which is floating above the disk surface due to the rotation of the disk, be 0.2 .mu.m or less. Hence, in order to realize this severe condition by means of the texture process, the height of the protrusions rising at the shoulders of the grooves must be suppressed at less than the distance by which the head floats up, so that it must be avoided for the head to collide against the protrusions. Thus, very strict surface characteristics of the disk is required. If only contact of the protrusions against the head sliding surface must be avoided, the head can be provided with a sufficient floating distance by changing the configuration of the head sliding surface, the leads applied to the head, the rotational speed of the disk, and so on. However, since the distance by which the head floats up must be reduced due to the increase in the recording density of the disk unit (it is ideal that the head is located as close to the magnetic film as possible), as stated above, and since the degree at which the protrusions deform and the degree at which the protrusions wear must be reduced, the height of the protrusions must be made uniform, and the area of the protrusions with which the head sliding surface makes contact must thereby be increased. Furthermore, deep pits must be provided in order to eliminate the debris of the surface of the disk.
Accordingly, it has conventionally been proposed to make the height of the protrusions formed as a consequence of formation of grooves uniform by dividing the polishing process subjected to the surface of the disk substrate into first and second processes and by polishing the protrusions in the second process. However, the optimal surface character of the disk substrate which is subjected to the texture process to the CSS characteristics and the head stickiness when the amount at which the head floats up is small, are unknown, and the problems involving the head crashing or the like have not yet been solved. More specifically, when the surface of the disk substrate is sufficiently polished to make the height of the protrusions uniform, the area of the surface of the disk against which the head slides (strictly speaking, the polished surface of the protrusions with which the head sliding surface makes contact) increases, thereby deteriorating the floating characteristics of the head. Furthermore, the lubricant coated on the surface of the disk (generally, a lubricant film is coated on the disk) or water contents contained in the air may attach to and accumulate on the head sliding surface due to the surface tension, and makes the head sliding surface adsorbed to the surface of the disk, causing cessation of rotation of the disk or damage to the head.
Japanese Patent Unexamined Publication No. 62-236664 discloses another method of forming fine irregularities on the recording surface of the magnetic disk. In the conventional methods disclosed by Japanese Patent Unexamined Publications Nos. 54-23294 and 62- 236664 the fine irregularities are formed on the disk substrate 30 in the circumferential direction by moving the resilient contact rollers 8 back and forth in the radial direction of the disk substrate 30 while pressing them against the rotating disk 1 through the polishing tapes 4 and, concurrently with this, by winding the polishing tapes 4, as shown in FIG. 12. Furthermore, the height of the irregularities is made uniform by conducting a second process on the surface which has been subjected to the above-described process as the first process using the polishing tapes on which abrasive grains having an average grain diameter smaller than that of the polishing tapes employed in the first process are fixed.
The above-described conventional techniques pay no sufficient attention to the accuracy with which the micro grooves are formed on the recording surface and have a disadvantage in that the pitch or height of the fine irregularities on the recording surface differs depending on the site of the recording surface. Furthermore, these techniques suffer from a problem in that the irregularities cannot be formed on the surface of the disk at a height or pitch required to satisfy the head flyability to the disk or the durability of the disk surface due to use of a non-uniform abrasive grains of the polishing tapes. The present inventors made experiments in which irregularities were formed on a Ni--P plated aluminum disk 17 shown in FIG. 9 in the manner shown in FIG. 12 using a polyester film polishing tape 4 to which aluminum oxide grains having a grain diameter of 3 .mu.m were fixed. The aluminum disk 17 had a surface roughness Ra of 2 to 3 nm, an outer diameter of 130 mm, an inner diameter of 40 mm, and a thickness of 2 mm. The experiments were conducted under a pressurizing force of 10 N, at a disk rotational speed of 400 rpm, at an elastic contact roller feed speed of 100 mm/min, and at a polishing tape feed speed of 100 mm/min. FIG. 43 shows the cross-section of the thus-obtained disk. As shown in FIG. 43, the height of the protrusions and the depth of the grooves were non-uniform. In the graph shown in FIG. 43, the axis of abscissa represents the radial direction of the magnetic disk, and the axis of ordinate represents the vertical direction of the irregularities. When a magnetic disk unit incorporating such a magnetic disk was driven, the magnetic head could not float stably and was damaged due to so-called head crash in which the magnetic head makes contact with the protrusions on the surface of the magnetic disk. Furthermore, when the magnetic head was caused to slide 1000 times against the disk, the lubricant film or the protective film formed on the magnetic disk was broken. Furthermore, the tangential force of the magnetic head increased, as shown by the curve 122 of FIG. 40, in proportion to the times with which the magnetic head was caused to slide, and reached about 0.1 N and the cessation of rotation of the magnetic disk thereby occurred when the magnetic head caused to slide 10,000 times. Hence, the surface of the disk was conventionally made level by conducting the second process on the surface of the disk, i.e., by polishing it again with the polishing tapes 4 of the smaller abrasive grains. However, even if the protrusions of the very non-uniform irregularities shown in FIG. 43 are polished to some degree by the second process, the aforementioned problems remain unsolved.
In addition to the above-described Japanese Patent Unexamined Publication Nos. 54-23294 and 62-248133, the method of forming a texture by providing fine irregularities on the substrate of the magnetic disk in order to improve durability of the disk surface and electrical characteristics is also disclosed in Japanese Patent Unexamined Publication No. 62-203748. In these texture forming methods, the texture is formed in the circumferential direction of the disk by using the polishing tape or free abrasive grains. In a case in which the polishing tapes are used, the texture is formed by the conventional disk manufacturing method shown in FIG. 12 by moving the contact rollers 8 back and forth in the radial direction of the disk 2 while pressing them against the rotating disk 2 through the polishing tapes 4 and, concurrently with this, by winding the polishing tapes 4. FIG. 55B shows the cross-section of the surface of the thus-obtained disk, which was measured along the direction indicated by the arrow in FIG. 55A.
However, the aforementioned conventional techniques give no attention to the accuracy of the shape, and have a problem in that the cross-sectional form of the surface of the disk differs depending on the position thereon. Furthermore, the above-described conventional techniques employ the polishing process and thus surface from problems in that debris of the surface is generated, in that scratches are generated and in that the processing residue remains.
Furthermore, since the aforementioned conventional techniques employ abrasive grains in the form of a polishing tape or the like, the height or pitch of the irregularities of the texture cannot be freely set, and it is therefore difficult to form irregularities that can satisfy the head floating characteristics, the contact start-stops characteristics and the electrical characteristics.